1. Field of Invention
This invention relates to a process for controlling tone reproduction of an output image and, more particularly, to a process for controlling tone reproduction in generating an output image by correlating a basic gradation transfer function and a plurality of sets of gradation coeffients to a set of visual sample images representing a closest visual reproduction and visual enhanced reproductions of an input image.
2. Description of Related Art
In monochrome half-tone image reproduction, various shades of gray appearing in a continuous tone original image are reproduced on paper by a single tone ink. To simulate the multiple shades of gray in the original, the continuous tone image is converted into a half-tone image. Half-tone images comprise a plurality of different size dots of a uniform optical density level. The human eye integrates the dots with the background and is fooled into believing that it sees a multiplicity of continuous tone gray shades, the apparent gray level being a function of the size of the dots per unit area.
Prior to the wide-spread use of computers, to make a half-tone image from a continuous tone one, a screening process was used. In that process the original was photographed, using high contrast photographic films known as litho films, to produce a half tone transparency. A large graphic arts camera was used, and the litho film was exposed through, preferably, a glass screen. The effect of the intervening screen was to produce an image on the litho film which was no longer a continuous tone image, but which consisted of a multiplicity of different size dots of substantially uniform density, the dot size being a function of the screen and optical image density at any point on the original.
The half-tone transparency is next used to make a printing plate. Typically this process involves exposing to radiant energy through the half-tone transparency a photosensitive oleophilic or hydrophilic layer coated on a support. As a result of this exposure, and subsequent development, the printing plate contains a pattern of dots representing the continuous tone image. This plate in turn is mounted on a printing press, the dots are inked, and the ink is transferred on a receiving sheet of paper or other material, reproducing the dot pattern and thus the image.
As can be readily understood, in the reproduction process, the dot sizes from the half-tone transparency through the inking and printing process often change in size and shape, depending on a number of variables, beginning with the degree collimation of the actinic radiation source used to expose the printing plate through the half-tone transparency, through the degree of contact between the separation and the plate, the amount of ink applied by a particular inking system in the press, the amount of pressure in the ink transfer step in the printing step and finally the nature, surface quality and absorbency of the receiving medium.
The task of properly selecting the dot sizes in the half-tone transparency is further complicated because it is often desired not to simply reproduce a print in which the gradation levels are an exact reproduction of the original, but to further adjust for desired tone scale reproduction. For instance, the operator is often required to enhance the original image as photographed, as a result of changes requested by a customer, to bring out certain details in the print or tone down others depending upon the desired effect.
A certain amount of flexibility in the half-tone transparency creation step was available to the operator in the form of variable exposure time. The result of such variable exposure was to produce smaller or larger dots than those expected through the correct exposure of the litho film, allowing the operator to compensate for the dot gain or loss in the subsequent printing steps and to introduce intentional alterations in response to a customer's instructions regarding print appearance. This process is one of trial and error based on operator experience and involved substantial experimentation.
Attempts at a scientific approach to predictably adjust the size of dots in the half-tone transparency to compensate for various printing conditions through the use of graphing techniques (e.g., developing various transfer curves correlating calculated dot size to original image density, and dot-gain prediction and/or measurement techniques (such as discussed in Appendices C and D of Principles of Color Proofing by Michael H. Bruno, published by GAMA Communications, Salem, N.H., 1986 edition, pages 333 et seq.) have met with limited success due in part to the multiplicity of variables introduced in the various steps described above.
With the advent of computers and the ability to handle images digitally, one now has the opportunity to generate half-tone transparencies through an electronic screening process in a computer system. While this technology allows a greater degree of flexibility in selecting the dot size produced on the half-tone transparencies, the selection of actual dot sizes to represent shades of gray with predictable results following printing in a particular printing set-up remains a problem.
In a typical digital image environment, there are three steps involved in generating the half-tone transparency. First, a continuous tone image is scanned in a scanner-digitizer. The scanner-digitizer breaks up the continuous tone image into a plurality of tiny individual picture elements (Pixels), each having an assigned numerical value related to the optical density of the picture element. Second, this numerical data is sent for processing in an image processing stage or system. The image processing system generates a driving signal. Third, a recorder receives and is driven by the driving signal which produces the transparency. In some applications, the recorder may produce a printing plate directly, omitting the formation of an actual transparency. In these applications, a virtual, rather than an actual, transparency exists in the form of numeric data supplied to the recorder. In other applications, the driving signal may produce an output image on a video display terminal.
There are a number of algorithms available in the art for generating half-tones from continuous tone data. U.S. Pat. No. 4,654,721 issued to Goertzel et al. and U.S. Pat. No. 4,667,250 issued to Murai provide good examples of this technology. Use of these algorithms will create a pattern of dots covering a desired percentage of a given unit area. The difficulty lies in determining what actual percentage dot coverage is needed to be created on the half-tone transparency per unit area for each of the optical densities in the original to accurately and predictably reproduce the original optical image density range or gamut, following printing, in a particular printing set-up. Further, before the data is processed by the half-tone algorithm and fed to the recorder, it must also be processed so that the printed image which will result from the use of the half-tone transparencies produced by the recorder either (1) will be an exact representation of the original continuous tone image as defined below or (2) will differ from the original by a preselected, predictable degree.
Known tone reproduction techniques which attempt to achieve this, follow several approaches. According to one of the approaches, a continuous tone image is scanned and an algorithm representing a preset generic half-tone density transfer function correlating half-tone transparency dot sizes and original densities based on preestablished non-user specific printing conditions is capable of being applied to the scanner output data to produce a print. A plurality of additional fixed preselected half-tone density transfer functions which produce prints differing from the print produced by the generic transfer curve are also available, which when applied to input data, produce half-tone transparencies that result in prints that differ from the original in appearance. The data may be corrected in real time, or may be stored in a memory for later access. The problem with such a system is that it lacks adaptability to particular press conditions which tend to vary from shop to shop and may be significantly different from the preset non-user specific conditions used in the generic half-tone density transfer function or algorithm applied.
As individual operator's printing conditions vary, there is need for extensive experimentation by each operator before the operator can use the preset settings to produce acceptable results with any degree of confidence. Furthermore, the operator must use his or her personal judgment and experience in choosing the particular gradation correction from those available, based on the original image quality and his or her printing conditions. The process is both time-consuming and expensive.
According to another approach, an existing system endeavors to employ a user-specific transfer algorithm or function which provides a half-tone transparency resulting in a print from a specific press and under specified printing conditions such that the print is an "exact" replica of the original continuous tone image. This method overcomes one of the major drawbacks of the previously described system. That is, it uses a transfer function which is user-specific. The system provides separate controls for selecting the basic transfer function and desired tone reproduction shifts. However, to achieve a desired altered tone reproduction from the original, this system still requires the operator to experimentally determine by how much to alter the basic transfer function each time to obtain a desired print appearance. The user is often at a loss to determine the proper adjustment of control settings to change the algorithm to accomplish a desired result since he has no guidance except his personal experience as to the effect of the various possible settings. Further, over the course of several jobs, it is often difficult to keep track of the series of changes made to accomplish a desired result should it be requested again.
In yet another approach, a system employs a set of user-specific basic transfer functions. Each one of the basic transfer functions provides a half-tone transparency resulting in a print under a different set of printing conditions such that the print is an "exact" replica of an original continuous tone image. In addition, a family of fixed preselected alternative transfer functions corresponding to each basic transfer function is available. Each one of the additional functions is adapted to produce a print differing in appearance from the print produced by the corresponding basic transfer function and the other additional functions corresponding to that basic transfer function. This system includes a user interface comprising a single tone reproduction control. All available basic transfer functions and additional associated transfer functions are grouped together and selected through this single control. Operators mentally decide their operating conditions and tone reproduction choices, then search through the vast number of options available on the single control. This is time consuming and confusing.
Thus, there is still need in the art for a method or process which will allow a user to produce an output image from an input image which will result in the output image having a predictable relation to the input image without undue experimentation or special knowledge.